1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to an in-plane switching mode liquid crystal display device and a method for manufacturing the same.
2. Discussion of the Related Art
A liquid crystal display device refers to a display apparatus in which a liquid crystal material having anisotropic permittivity is provided between a color filter (CF) substrate and a thin film transistor (TFT) array substrate that are upper and lower transparent insulating substrates. An intensity of an electric field provided in the liquid crystal material is controlled such that a molecular arrangement of the liquid crystal material is changed, and an amount of light transmitted to the transparent insulating substrate through the liquid crystal material is controlled, thereby displaying a desired image. In the liquid crystal display device, a thin film transistor liquid crystal display (TFT LCD) using a thin film transistor (TFT) as a switching element is mainly used.
An electro-optical characteristic of a liquid crystal cell is greatly dependent on formation of an alignment film and a surface treatment state based on rubbing. The alignment film is provided on an uppermost layer of the TFT array substrate and the color filter (CF) substrate, and its surface is rubbed to arrange liquid crystal molecules in a particular direction.
In multidomain vertical alignment (MVA) and patterned vertical alignment (PVA) technologies that are vertical alignment (VA) mode technologies using vertical alignment, a negative dielectric liquid crystal is used to align the liquid crystal molecules perpendicularly to the alignment film. Therefore, the alignment film is not rubbed. However, in a twisted nematic mode liquid crystal cell, alignment films of a TFT array substrate and a CF substrate have rubbing directions perpendicular to each other. For example, in order to provide a characteristic of an optimal viewing angle based on a front and down direction, the TFT array substrate is rubbed from a left and up direction to a right and down direction on the basis of a screen of a liquid crystal panel, and the CF substrate is rubbed from a left and down direction to a right and up direction.
FIG. 1 is a sectional view simply illustrating an in-plane switching mode liquid crystal display device according to the related art.
In FIG. 1, the in-plane switching mode liquid crystal display device comprises a TFT array substrate 10 and a color filter substrate 20 spaced apart from each other and sealed, and a liquid crystal (LC) injected therebetween. First and second polarization plates 30 and 31 are provided on a bottom of the TFT array substrate 10 and a top of the color filter substrate 20, respectively.
A common electrode 11 and a pixel electrode 12 are spaced apart and provided at a pixel region of the TFT array substrate 10 on the same plane. In-plane switching based on a horizontal direction that is provided as voltage is applied to the common electrode 11 and the pixel electrode 12 drives a liquid crystal (LC).
First and second alignment films 13 and 21 are provided over the TFT array substrate 10 and define an initial alignment of the liquid crystal (LC). The first and second alignment films 13 and 21 are rubbed at a pretilt angle of about 1.5° so that the initial alignment of the liquid crystal (LC) is aligned in a horizontal direction of a substrate surface.
An in-plane switching mode liquid crystal display device according to the related art is in a normally black state, and does not allow light transmission before voltage is applied. If voltage is applied to the common electrode 11 and the pixel electrode 12, an electric field is provided between the two electrodes 11 and 12 and the liquid crystal (LC) is aligned depending on the electric field provided between the two electrodes 11 and 12. After voltage is applied, internal light is transmitted depending on the alignment of the liquid crystal (LC), thereby displaying a white state. The liquid crystal (LC) has positive (+) dielectric anisotropy, and has a characteristic in which a long axis is aligned in an electric field direction.
In an off-state in which in-plane switching mode is applied to the common electrode 11 and the pixel electrode 12, the alignment direction of the liquid crystal (LC) does not change. In comparison to this, in an on-state in which the in-plane switching mode is applied, the alignment direction of the liquid crystal (LC) is changed, and the liquid crystal (LC) is aligned at a twisted angle of about 45° in comparison to the off-state.
FIGS. 2A and 2B are reference views illustrating the rubbing direction of the horizontal alignment film, and the alignment of the liquid crystal based thereon in FIG. 1, illustrate a side view and a top view, respectively.
Polarization axes (POL and ANAL) of the first and second polarization plates 30 and 31 are perpendicular to each other, and are associated with rubbing directions (alignment processing directions) of the first alignment film 13 and the second alignment film 21.
In the in-plane switching mode, the first and second alignment films 13 and 21 of FIG. 1 are generally formed using the horizontal alignment film. In this case, near an electrode having a step such as the common electrode 11 or the pixel electrode 12, an alignment state of the liquid crystal (LC) is provided as shown in FIGS. 2A and 2B.
However, if the horizontal alignment film is used in the in-plane switching mode, rubbing is not performed due to an electrode edge part at a side of a rubbing direction. Accordingly, as shown in FIGS. 2A and 2B, in a step coverage region (R1), arrangement of the liquid crystal is not uniform, and the region does not have anisotropy in comparison to other regions in which the rubbing is performed.
As such, in the horizontal alignment film, if the rubbing is not performed, the alignment of the liquid crystal is remarkably deteriorated and, for example, there is a drawback in that, when black is embodied, the region generates phase delay due to non-uniform rubbing and therefore, light leakage is caused and luminance is increased, thereby not clearly embodying the black.